There have been various technological approaches to produce a writing device as a replacement of paper and pencil or chalk on slate. The best known examples are toys. The ETCH-A-SKETCH™, introduced in the 1960s, is one such device. In this device, a movable stylus removes a powder material from inside a screen to make a dark line. The image is erased by turning the device upside down and shaking it to smooth out the surface. Another famous example is the MAGNA DOODLE™, which is a magnetophoretic device in which a stylus with a magnet on the tip is used as the pen to draw a line. The device is erased with a thin long magnet behind the screen. Over 40 million of these devices have been reportedly sold.
Other liquid crystal writing devices have also been proposed. U.S. Pat. No. 4,525,032 to Hilsum is one such example where cholesteric or a smectic liquid crystal is used to provide a semi-permanent record of the path traced by a stylus on a display and used as a re-usable writing pad. According to Hilsum a layer of a liquid crystal material is contained between two substrates. A stylus having a tip contacts the front substrate and changes the state of selected areas of the liquid crystal layer at positions adjacent the pen tip to provide observable information corresponding to the pen movement. The pen may have a pointed tip, a heated tip, a light emitting tip, or a tip connected to a high voltage high impedance source. At least one substrate of the display can be deformable, thin, or flexible so that the liquid crystal layer may be changed from one state to another by localized application of pressure, heat, light, electrostatic charge, or an electric field. The resultant image on the display is erased by deformation of the layer, e.g. flexing, heating and cooling, or by an electrical field.
A practical problem with the Hilsum device is erasing the image. It is slow and inconvenient to heat or flex the device to erase the image. Hilsum discloses an electronic means of erasure using a special cholesteric liquid crystal in which the frequency of an AC field is applied to the stylus or electrodes. The frequency is changed to enable a writing state or an erasure state. However, this is not without problems in that crossover frequency between writing and erasing is strongly temperature dependent and the frequencies as well as the voltages are very high, consuming a lot of power causing very limited battery lifetime.
A considerable improvement was made with the discovery of bistable cholesteric liquid crystals (see U.S. Pat. No. 5,453,863). Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. These materials possess a helical structure in which the liquid crystal (LC) director twists around a helical axis. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. They can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth by controlling the pitch of the helical twist through the concentration of chiral dopants and the birefringence of the nematic host, respectively. If the incident light is not polarized, it will be decomposed into two circular polarized components with opposite handedness and one of the components reflected.
The cholesteric material is typically electrically switched to either one of two stable textures; planar or focal conic as described, for example, in the U.S. Pat. No. 5,453,863. In the planar texture, the director of the LC (direction of the long axis of the molecule) is uniformly parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high; thus the helical domain size becomes small and randomized in orientation such that it is just weakly scattering and does not reflect impinging light (i.e., it is substantially transparent to incident light). Once the defect structures are created, they are topologically stable and cannot be removed unless by some external force such as an electric field or melting the material out of the liquid crystalline phase to the isotropic. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. These bistable structures can be electronically switched between each other. Gray scale is also available within a single pixel through various switching schemes in order to adjust the density of reflective helical domains that are oriented perpendicular to the substrates (planar texture) to the randomized forward scattering domains (focal conic texture).
In a cholesteric liquid crystal display (LCD), the liquid crystal is typically sandwiched between two substrates that are spaced to a particular gap. The substrates can be either glass or polymer. The bottom substrate is painted with a light absorbing (black or colored) background. The cell gap is usually set by polymer or glass spacers that are either cylindrical or spherical in shape. In most cholesteric liquid crystal displays, the cell gap is not intentionally changed. If one presses on the top substrate of the cholesteric LCD, the liquid crystal can be displaced (since fluids are not very compressible) and induced to flow radially out of the area. Of principle interest is that when the focal conic texture of the cholesteric liquid crystal is induced to flow, the resulting texture is the planar state. The reflective planar state contrasts well to the dark focal conic background. This is a principle behind U.S. Pat. No. 6,104,448 “Pressure Sensitive Liquid Crystalline Light Modulating Device and Material,” incorporated herein by reference in its entirety, which discloses that application of a mechanical stress to the liquid crystalline light modulating material changes an initial light scattering focal conic texture to the light reflecting planar texture. U.S. Pat. No. 6,104,448 discloses a polymer network that is soluble with the chiral nematic liquid crystal and phase separates to form separated polymer domains that stabilize the thickness of the cell structure.
In U.S. Pat. No. 6,104,448, an image can be written on the device with an untethered stylus or fingernail. The entire image is erased with the push of a button that applies a low voltage DC pulse to the cholesteric device. An advantage of bistable cholesteric materials is that an image created on the writing device does not degrade with time and lasts indefinitely without application of an electric field, until erased. However, use of a low voltage DC pulse to erase a cholesteric device as disclosed in U.S. Pat. No. 6,104,448 leaves the device susceptible to ghosting, in which the image to be erased is still faintly visible even after the erase has been completed. The voltage level of the pulse must also be accurate, as too low of a voltage results in some of the bright planar domains in the image remaining in the bright planar texture, and too high of a voltage results in homeotropically aligning some of the cholesteric material such that it goes to the bright planar texture upon removal of the pulse.
Prior art erase waveforms for a writing/drawing surface containing a cholesteric liquid crystal display or LCD (e.g., the erase waveforms used in a Boogie Board® eWriter by Kent Displays, Inc.) which address problems with ghosting are shown in FIG. 1A. The eWriter (liquid crystal writing device) of FIG. 1A contains two inputs designated A and B, into which drive waveforms VA and VB are applied, respectively, in order to erase the display to the dark focal conic state. The drive waveforms, VA and VB, consist of 3 levels: 0V, VFC (voltage focal conic) and VP (voltage planar). While each of these levels is zero or positive, the resultant voltage waveform seen across the display (VA-VB), is bipolar and consists of the levels 0V, ±VFC, and ±VP. Typical durations for the pulses (positive or negative) in the resultant waveform are 150 milliseconds (ms), with 50 ms between pulses.
The ±VP (voltage planar) pulses drive the liquid crystal to the homeotropic state from both the planar and focal conic initial textures in order to clear away the previous image. By themselves, these quickly turned off (discharged) pulses would leave the display in the bright (reflecting) planar texture. The ±VFC pulses are required to put the display in the dark (nonreflecting) focal conic texture in preparation for new writing/drawing. The time between pulses has the function of creating added turbulence within the liquid crystal to help eliminate ghosting of the previous image. Another function of the time between pulses is reducing power consumption by discharging the display prior to applying a pulse of opposite polarity. The DC-balance of the resulting waveform enables the display to undergo continual, repeated switching without failures which might otherwise occur due to ionic impurities within the display.
FIG. 2 provides a schematic representation of the display drive circuitry used to create the waveform of the prior art. The cholesteric liquid crystal display (LCD) writing/drawing surface 200 (represented internally as a capacitor) has two drive terminals 210 and 220. High-side drivers 250 are provided to connect the drive terminals 210 and 220 to a high voltage supply 240. Low-side drivers 260 are provided to connect the drive terminals 210 and 220 to ground. Current-limiting resistors 230 (optional) are included between drive terminals 210 and 220 and drivers 250 and 260. Control signals 270 through 273 are sequenced to enable/disable the drivers in order to produce the required drive waveforms at drive terminals 210 and 220. In coordination, high voltage supply 240 is configured to output VP during the first two pulses and VFC for the remaining pulses.
The prior art for erasing the writing/drawing surface relies on approaches originally developed for glass-based cholesteric LCD signage products. The displays in these products required DC-balanced drive waveforms with precise voltage levels in order to effectively erase, with the drive waveforms often producing a visible flash on the display during the erase.
It would be useful to produce electronic liquid crystal writing devices with erase functionality developed specifically for the flexible cholesteric liquid crystal writing/drawing surface of these devices. Such devices would benefit from reduced size, complexity, and cost in the boost converter and drive circuits, while also improving energy efficiency and eliminating visible display flashes during erase.